HERITABLE VARIATION IN RESOURCE UTILIZATION AND RESPONSE IN A WINERY POPULATION OF DROSOPHILA MELANOGASTER

It has been found that Drosophila melanogaster lines from the “Chateau Tahbilk” winery cellar had higher larval ethanol resistance than lines originating from outside the cellar. Because the adaptive significance of this trait is unclear, we have reexamined genetic microdifferentiation at Tahbilk with other resources and different tests for ethanol adaptation. Cellar stocks tended to be more resistant to starvation after exposure to wine seepage than stocks originating from orchard traps outside the cellar. Lines from a grape residue pile were also more resistant to starvation after seepage exposure than orchard stocks even though these collection sites were a few meters apart. Cellar and orchard stocks did not differ in ethanol resistance as measured by larval viability tests on low sucrose medium. However, stocks from the grape residue pile showed an increase in adult longevity over ethanol vapor compared to those from the cellar or orchard stocks. These differences were not due to maternal effects. In laboratory tests of behavioral responses, cellar stocks were relatively more attracted to seepage than orchard stocks in one year but not in the other two years. The findings suggest some adaptive differentiation related to resource heterogeneity at Tahbilk.     

CONTINUING STUDIES ON THE SOUTH AMHERST DROSOPHILA MELANOGASTER NATURAL POPULATION DURING THE 1970'S AND 1980'S

Continuing investigations on the South Amherst Drosophila melanogaster natural population following the significant decline and recovery of lethal (le) and semilethal (sle) frequencies in the late 1960's (Ives, 1970) show that the population has been remarkably stable although it contains MR (male recombination) and/or P DNA elements (Kidwell et al., 1977a; Green, 1980). A 13-year study affirms that the lethals present are nonrandomly distributed along the second chromosome and deficient on the right; they differ significantly in distribution from spontaneous (Ives, 1973) and δ-induced lethals (Minamori and Ito, 1971). Between 1970 and 1977, a total of 4,083 second chromosomes from the Markert subpopulation were analyzed; 28.9% of the chromosomes were lethal and 7.25% were semilethal in homozygous condition. Frequencies are similar for early summer and late fall collections although the rate of allelism among lethals is significantly higher in early summer than in late fall. For the large fall (1970–1979) Porch site population, 2,519 second chromosomes were analyzed; 29.5% were lethal and 8.0% were sublethal as homozygotes; the rate of allelism among lethals was 1.50%. At Hockanum, 1977–1983, lethal and semilethal frequencies were lower; the rate of allelism among lethals was 1.43%. The chromosome map distribution of lethals does not change between summer and late fall at Markert. The overall distributions of lethals at the Markert and Hockanum sites are similar. In tests for male recombination (MR) activity in the population over a 6-year period, a total of 0.47% recombinants were observed; these were uniformly distributed along the second chromosome. Comparisons are made with other long studied D. melanogaster populations.     

GENETIC COMPONENTS OF VARIATION IN ENERGY STORAGE IN DROSOPHILA MELANOGASTER

One approach to examining the underlying genetic structure of the variation in a continuous phenotype is to measure a set of possibly mechanistically related traits and determine the quantitative genetic aspects of their transmission. In this study the quantities of stored triacylglycerol and glycogen were measured along with the activities of 10 enzymes in related metabolic pathways in a set of 1,157 half-sib families of Drosophila melanogaster. The families were structured with each male being mated to 10 females and two offspring were scored from each female. Parents and offspring were scored for the phenotypes, and the components of variance (additive, dominance, and environmental) were estimated in three ways, including analysis of variance on offspring alone, parent-offspring regression, and maximum likelihood methods. While there were differences among the estimates made by the three methods, a consistent result was that substantial additive genetic variation was detected for all the traits. Consistent with models for the quantitative genetics of enzyme kinetics, the genetic variances of global properties were largely additive. Previous studies with extracted chromosome lines had indicated several significant genetic correlations among these characters, and much of the correlation was attributable to additive effects. The results imply that there is substantial opportunity for natural or artificial selection to act on quantities of stored lipid and carbohydrate, and that the response to selection is likely to be in part mediated by changes in the kinetics of the enzymes targeted in this study.     

NATURAL VARIATION IN THE EXPRESSION OF THE HEAT-SHOCK PROTEIN HSP70 IN A POPULATION OF DROSOPHILA MELANOGASTER AND ITS CORRELATION WITH TOLERANCE OF ECOLOGICALLY RELEVANT THERMAL STRESS

Although Hsp70, the principal inducible heat-shock protein of Drosophila melanogaster, has received intense scrutiny in laboratory strains, its variation within natural populations and the consequences of such variation for thermotolerance are unknown. We have characterized variation in first-instar larvae of 20 isofemale lines isolated from a single natural population of D. melanogaster, in which larvae are prone to thermal stress in nature. Hsp70 expression varied more than twofold among lines after induction by exposure to 36°C for one hour, with an estimated proportion of the variation due to genetic differences of 0.24 ± 0.08. Thermotolerance with and without a Hsp70-inducing pretreatment, survival at 25°C, and developmental time also varied significantly. As expected, expression of Hsp70 correlated positively with larval thermotolerance. By contrast, lines in which larval survival was high in the absence of heat stress showed lower than average Hsp70 expression and lower than average inducible thermotolerance. This conditional performance suggests an evolutionary trade-off between thermotolerance and the ability to produce higher concentrations of Hsp70, and survival in a benign environment.     

GENETIC BASIS OF A BETWEEN-ENVIRONMENT TRADE-OFF INVOLVING RESISTANCE TO CADMIUM IN DROSOPHILA MELANOGASTER

In a replicated, laboratory, natural selection experiment Drosophila melanogaster populations were maintained for 20 generations either on unpolluted medium or on polluted medium containing cadmium chloride at a concentration of 80 μg/ml. Lines maintained on polluted medium evolved resistance. In comparison with unpolluted lines, their juvenile survivorship increased from 35% to 46%, developmental period decreased from 13.7 days to 13.0 days, and fecundity increased from 3 to 29 eggs per two-day period. Emergence weights, however, did not change. By contrast the “environmental” effect of moving susceptible flies onto polluted medium was that after two generations survivorship fell 62%, developmental period increased 40%, and fecundity fell 97%. Emergence weights fell 31% in females and 28% in males. Resistant lines paid a fitness cost in unpolluted environments, with fecundity being reduced by 44% and emergence weights being reduced by 4% in females and 6% in males. Developmental period, however, was unaffected. Analyses of crosses and backcrosses between the lines suggested that the evolved cadmium resistance was due to a single sex-linked gene. Levels of dominance were calculated, and in each life-history character the resistant allele was found to be completely dominant. Because the life-history effects appear to be produced by a single gene, it is probable that they all depend on the same metabolic pathway. Metallothionein production is a likely candidate because this is known to be controlled by genes on the X-chromosome. The study adds to a small number of examples of single or closely linked genes with large antagonistic pleiotropic effects on life histories. The result here is a between-environment trade-off, allowing animals increased fitness in polluted environments, but only at the cost of reduced growth and reproduction in unpolluted environments.     

MULTIPLE GENETIC MECHANISMS FOR THE EVOLUTION OF SENESCENCE IN DROSOPHILA MELANOGASTER

We present the results of selection experiments designed to distinguish between antagonistic pleiotropy and mutation accumulation, two mechanisms for the evolution of senescence. Reverse selection for early-life fitness was applied to laboratory populations of Drosophila melanogaster that had been previously selected for late-life fitness. These populations also exhibited reduced early-age female fecundity and increased resistance to the stresses of starvation, desiccation, and ethanol, when compared to control populations. Reverse selection was carried out at both uncontrolled, higher larval rearing density and at controlled, lower larval density. In the uncontrolled-density selection lines, early-age female fecundity increased to control-population levels in response to the reintroduction of selection for early-age fitness. Concomitantly, resistance to starvation declined in agreement with previous observations of a negative genetic correlation between these two characters and in accordance with the antagonistic-pleiotropy mechanism. However, resistance to stresses of desiccation and ethanol did not decline in the uncontrolled-density lines during 22 generations of reverse selection for early-life fitness. The latter results provide evidence that mutation accumulation has also played a role in the evolution of senescence in this set of Drosophila populations. No significant response in early-age fecundity or starvation resistance was observed in the controlled-density reverse-selection lines, supporting previous observations that selection on Drosophila life-history characters is critically sensitive to larval rearing density.     

INVERSION POLYMORPHISM,LONGEVITY, AND BODY SIZE IN A NATURAL POPULATION OF DROSOPHILA BUZZATII

In this study we present the results of an analysis of differential longevity associated with Drosophila buzzatii second chromosome inversion karyotypes based on the assessment of more than 1000 individuals collected in a natural population. Comparisons of inversion frequencies between emerged and bait-collected flies showed not only that inversion arrangements were associated with differential longevity, but also that selection was sex specific. Because each individual fly was scored for thorax length and karyotype, we were able to show that longevity selection favoring larger flies coupled with the average effect of inversions on thorax length can account for the change of inversion frequencies due to longevity in females. The observed genotypic-by-sex interaction could be an important mechanism involved in the maintenance of the polymorphism. Arrangement 2Jz³, which was shown to impaired fecundity in two independent previous studies, exhibited a positive effect on longevity. This pattern of negative pleiotropy may be another plausible mechanism accounting for the maintenance of the polymorphism.     

RESISTANCE OF GENETIC CORRELATION STRUCTURE TO DIRECTIONAL SELECTION IN DROSOPHILA MELANOGASTER

The genetic covariance and correlation matrices for five morphological traits were estimated from four populations of fruit flies, Drosophila melanogaster, to measure the extent of change in genetic covariances as a result of directional selection. Two of the populations were derived from lines that had undergone selection for large or small thorax length over the preceding 23 generations. A third population was constituted using flies from control lines that were maintained with equivalent population sizes as the selected lines. The fourth population contained flies from the original cage population from which the selected and control lines had been started. Tests of the homogeneity of covariance matrices using maximum likelihood techniques revealed significant changes in covariance structure among the selected lines. Prediction of base population trait means from selected line means under the assumption of constant genetic covariances indicated that genetic covariances for the small population differed more from the base population than did the covariances for the large population. The predicted small population means diverged farther from the expected means because the additive genetic variance associated with several traits increased in value and most of the genetic covariances associated with one trait changed in sign. These results illustrate that genetic covariances may remain nearly constant in some situations while changing markedly in others. Possible developmental reasons for the genetic changes are discussed.